1. Field of the Invention
The present invention relates to a photothermographic material and an image forming method using the same.
2. Description of the Related Art
In recent years, decrease in the amount of processing liquid waste in the field of films for medical imaging has been keenly desired from the viewpoints of environmental protection and economy of space. For this reason, techniques regarding photothermographic materials for medical diagnosis and graphic arts, which can be exposed efficiently by laser image setters or laser imagers and can form clear black-toned images of high resolution and sharpness, is required. According to the photothermographic materials described above, thermal development systems which do not require liquid processing chemicals, are simpler, and do not damage the environment can be supplied to customers.
While similar requirements also exist in the field of general image forming materials, images for medical imaging require a particularly high image quality excellent in sharpness and granularity since fine representation is required, and are characterized in that images of blue-black tones are preferred from the viewpoint of easy diagnosis. At present, various kinds of hard copy systems utilizing dyes or pigments such as ink jet printers and electrophotographic systems have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are described in a number of documents. A photothermographic material generally comprises an image forming layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toning agent for controlling the color tone of silver, dispersed in a binder. A photothermographic material forms a black silver image by being heated to a high temperature (for example, 80° C. or higher) after image wise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region. Further, the Fuji Medical Dry Imager FM-DP L is has been marketed as a medical image forming system using a photothermographic material.
Thermal image forming systems utilizing organic silver salts can be produced by solvent application, as well as by applying and drying a coating solution that contains an aqueous dispersion of polymer particles as a main component of a binder. The latter method does not require an operation of, for example, recovering a solvent, and therefore can be carried out with simple production facilities and is environmentally friendly. Accordingly, the latter method is advantageous for mass production. However, since the coating solution does not have setting ability, a coated layer may be turbulent due to an air flow for drying the applied coating solution, resulting in non-uniformity of the dried layer.
Use of a hydrophilic binder such as gelatin as the binder has been proposed (see, for example, U.S. Pat. No. 6,713,241 and Japanese Patent Application Laid-pen (JP-A) No. 2004-110038). However, such methods have problems such as difficulty in obtaining high image density, a high degree of fogging, and unsatisfactory color tone of images.
A layer in a photothermographic material needs to contain chemical components necessary for forming an image, in advance. These chemical components may affect storage stability before use of the photothermographic materials. Further, even after an image has been formed through thermal development on a photothermographic material, such chemical components remain in the layer thereof in an unreacted state or in the form of reaction products. These chemical components remaining in the layer may affect transparency of the layer and color tone of the image, as well as preservability of the resulting image, and particularly, may cause an increase in fog due to “printout”.
Image processing techniques in which image information is digitized and necessary information is outputted at a terminal through communication, have been developed and widely used in the field of medical images and printing industries. In many cases, semiconductor lasers are used for outputting image information. With progress in practical application of blue lasers, high-resolution image systems using blue lasers are attracting attention. Conventionally, silver halide photosensitive materials for wet development have been used. However, there have been strong demands for developing a highly-sensitive photosensitive material for dry development that can record an image with blue-laser beam.